Since you repeated length two times, I'm assuming you are finding the width of the tank. The cubic feet (volume) = The length of the tank (length) * the width of the tank (width).
Let v = the cubic feet
Let l = length of the tank
Let w = width of the tank
So, v = l*w, this is equal to the first sentence I put, but just replaced with variables.
Next, determine given values.
v = 1004.5 cubic feet
l = 7 feet
w = ? <- this is what we need to find.
Substitute in all the known values.
1004.5 = 7 * w ( the variable w is used for undetermined width )
Divide by 7 on both sides to get x alone in the form of w = #.
w = 143.5
The width of the tank is 143.5 feet.
